1. Field of the Invention
The present invention relates to an electrically conductive adhesive used for mounting of electronic components and formation of wiring of an electronic circuit board.
2. Description of the Related Art
Heretofore, a Sn—Pb solder material, particularly a Snip eutectic solder material with a 63Sn-37Pb eutectic composition (composition of 63% by weight of Sn and 37% by weight of Pb) has conventionally been used as a bonding material for electronic component mounting.
However, in electronic component mounting, there has recently been required an improvement in the mechanical strength of the soldering portion and an improvement reliability of a thermal shock strength. While global environmental protection has become of major interest, regulation of treatment of industrial waste such as electronic circuit boards is becoming more severe and lead is becoming a global regulation subject.
Thus, in the bonding material, there is a tendency that the Sn—Pb solder material is replaced with a solder material containing no lead, a so-called lead-free solder material. Typical example of such a lead-free solder containing two kinds of metals as main components includes a Sn—Ag solder as a eutectic alloy material having excellent wettability (refer to Japanese Patent No. 3,027,441).
Since the melting point of the Sn—Ag solder is about 30 to 40° C. higher than the melting point of the Sn—Pb solder (about 183° C.), the soldering temperature in the case of using the Sn—Ag solder is higher than that in the case of using the Sn—Pb solder. Therefore, when the Sn—Ag solder is used, the mounting temperature in the case of mounting electronic components may become higher than the heat-resistant temperature of electronic components. In this case, there arises a problem that electronic components may be damaged.
As described above, since the melting point of the solder may exert an influence on the mounting temperature of the solder, a lead-free solder capable of lowering the melting point has been developed. Furthermore, an electrically conductive adhesive comprising a metal component of a lead-free solder and a resin component, particularly a thermosetting resin component has been developed. When using an electrically conductive adhesive in which both the melting point of the metal component and the curing temperature of the thermosetting resin component are comparatively low, damage caused by heat of the electronic component can be prevented or reduced upon mounting of electronic components. Thus, intense interest has been shown towards such an electrically conductive adhesive as a bonding material to replace the solder (refer to Japanese Unexamined Patent Publication (Kokai) No. 10-163605).
A general electrically conductive adhesive is formed by mixing filler particles of a metal component of a lead-free solder in either a bulk or flake form, with a thermosetting resin component in a predetermined ratio. When an electronic component is bonded with a circuit board using such an electrically conductive adhesive, volume resistivity may increase as compared with the case of bonding using a lead-free solder as a bulk metal or volume resistivity may sometimes vary even if an electrically conductive adhesive with the same composition is used. Therefore, use of the electrically conductive adhesive was restricted.
When electronic components are bonded using an electrically conductive adhesive containing Ag filler particles as a metal component, Ag filler particles may cause migration and/or sulfuration according to the environment using the electronic component.
In the case of the electrically conductive adhesive containing Ag filler particles as the metal component, Ag filler particles constitute a main electrically conductive component of the electrically conductive adhesive. Therefore, in either case when Ag filler particles cause migration or sulfuration, resistivity of the bonding portion or wiring formation portion increases. This causes a decrease in reliability of the electronic component with a lapse of time, thus leading to a short lifetime of the electronic component and electrical/electronic equipment using the electronic component. Therefore, it has been required to prevent migration and sulfuration of metal filler particles, particularly Ag filler particles.